On-line DOI rebinning for LSO PET/SPECT spatial resolution

Abstract

A method and apparatus for on-line DOI rebinning for LSO PET/SPECT to improve spatial resolution, for use in a hybrid Positron Emission Tomography (PET)/Single Photon Emission Computed Tomography (SPECT) system running in PET-mode. Data acquisition hardware is used to feed a detector pair coincidence event stream to an on-line rebinner. Gamma centroid location measurements are made by rastering assumed transaxial and radial head positions and the corresponding rebinning maps for optimal back-projected image resolution. Optimal positions are found by collecting a 64-bit list mode file, assuming a crystal position as the centroid for each of the heads, defining a sequence for varying the assumed positions, making the rebinning look-up tables, rebinning the list mode data, histogramming and reconstructing the image, assessing the image resolution, recording the best resolution number and the associated trial position variables, repeating these on the next trial variable set.

1. A method for improving spatial resolution using on-line depth-of-interaction (DOI) rebinning in a hybrid Positron Emission Tomography (PET)/Single Photon Emission Computed Tomography (SPECT) system running in PET-mode, the PET/SPECT system having a rotating dual-head tomograph using LSO/NaI scintillators and being capable of depth-of-interaction (DOI) measurement, the tomograph defining a field of view (FOV) within a patient gantry, said method comprising the steps of:
 (a) acquiring a PET coincidence data stream from a patient scan;  
 (b) delivering the PET coincidence data stream to an on-line rebinner to derive a bin address output stream;  
 (c) deriving a bin address output stream in said on-line rebinner from said PET coincidence data stream;  
 (d) delivering the bin address output stream from the rebinner to a Fibre Channel PCI DMA receiver card for direct PCI DMA stream transfers;  
 (e) histogramming the bin address output stream; and  
 (f) storing said bin address output stream.  
 
     
     
       2. The method of  claim 1  wherein said on-line rebinner is integrated within a rebinner circuitry which supports on-line real-time DOI line of response (LOR)-to-projection-space nearest-neighbor rebinning, and wherein DOI and gamma interaction centroid depth knowledge about LOR positioning is applied in real time. 
     
     
       3. The method of  claim 1  wherein said step of deriving said bin address output stream includes performing a transaxial computation to determine at least a sinogram angle and a radial offset of a coincidence event between a first detector having coordinates (x A , y A ) of a first detector head and a second detector having coordinates (x B , y B ) of a second detector head, wherein said sinogram angle is determined by the equation: 
         sa   =       tan     -   1       ⁡     [         y   b     -     y   a           x   b     -     x   a         ]         ,       
 
       and where said radial offset is determined by the equation:
     ro =√{square root over (( x   a   2   +y   a   2 ))}* sin(tan −1 ( y   a   /x   a )− sa).    
 
     
     
       4. The method of  claim 1  further comprising the step of measuring a Gamma centroid location including the steps of:
 (1) rastering assumed transaxial and radial head positions and a corresponding rebinning map for optimal back-projected image resolution, said step rastering using variables r A , t A , r B , t B  to represent radial and transaxial offsets for each head of the dual-head tomograph; and  
 (2) stepping the values for r A , t A , r B , t B  discretely through a chosen range, with all combinations of the variables being tested.  
 
     
     
       5. The method of  claim 1  further comprising the step of finding an optimal position of each head of the dual-head tomograph including the steps of:
 (1) collecting a 64-bit list mode file;  
 (2) assuming a crystal position as the centroid for each head; and  
 (3) defining a sequence for varying the assumed crystal positions.  
 
     
     
       6. The method of  claim 5  wherein said sequence for varying the assumed crystal positions includes the steps of:
 (i) making at least one rebinning look up table;  
 (ii) rebinning list mode data;  
 (iii) histogramming and reconstructing the image;  
 (iv) assessing image resolution using Full Width Half Maximum (FWHM) resolution;  
 (v) recording a best resolution number and an associated trial position variable; and  
 (vi) repeating said steps of rebinning list mode data through said step of recording a best resolution number and an associated trial position variable on a subsequent trial variable set.  
 
     
     
       7. The method of  claim 6 , in said step of assessing image resolution, wherein all FWHM resolution numbers are assessed from a conventional 2-D filtered back projection of the sinogram using a ramp filter with a 0.5 cutoff. 
     
     
       8. The method of  claim 6 , in said step of assessing image resolution, wherein both image horizontal and vertical FWHM assessments are performed. 
     
     
       9. The method of  claim 6  further comprising the step of defining a final rebinning look-up table to be applied for general use, including for LOR which do not serve the FOV center. 
     
     
       10. A method for improving spatial resolution using on-line depth-of-interaction (DOI) rebinning in a hybrid Positron Emission Tomography (PET)/Single Photon Emission Computed Tomography (SPECT) system running in PET-mode, the PET/SPECT system having a rotating dual-head tomograph using LSO/NaI scintillators and being capable of depth-of-interaction (DOI) measurement, the tomograph defining a field of view (FOV) within a patient gantry, said method comprising the step of:
 finding multiple gamma interaction centroid locations which are unique for different line of response (LOR) incident angles.